Principles of terrestrial ecosystem ecology.pdf

Figure 6.9. Representative seasonal pattern of gross
primary production, ecosystem respiration, and net
ecosystem production of an ecosystem. NEP is the
difference between two large fluxes (carbon inputs
in GPP and carbon losses, of which R ecosyst and Fleach
are generally greatest).Annual CO2 flux in this graph
is at steady state because the NEP summed over the
annual cycle is close to zero. Here, carbon losses due
to disturbance are assumed to be zero.
released to the atmosphere (see Chapter 10).
Approximately 20% of the CO 2 produced in
arctic soils, for example, leaches to groundwater
and is released from lakes and streams
(Kling et al. 1991). Dissolved organic carbon
is also lost from ecosystems by leaching to
groundwater. Despite their importance, leaching
losses of carbon to groundwater are seldom
measured and therefore frequently ignored in
ecosystem carbon budgets.
Lateral Transfers
Lateral transfer of carbon into or out of ecosystems
can be important to the long-term carbon
budgets of ecosystems. Carbon can move laterally
in ecosystems through erosion and deposition
by wind or water or by movement by
animals (Fig. 6.8). These lateral transfers are
usually so small that they are undetectable in
any single year. Over long time periods or
during extreme events, such as floods or landslides,
these lateral transfers can, however, be
quantitatively important. These transfers are
typically more important for elements that are
tightly cycled within ecosystems and have small
annual inputs and losses (e.g., phosphorus) than
they are for carbon.
Disturbance
Net Ecosystem Production 145
Disturbance is an episodic cause of carbon loss
from many ecosystems. Disturbances such as
fire and harvest of plants or peat can be the
dominant avenues of carbon losses from ecosystems
in the years when they occur. In many
cases the carbon losses with disturbance are so
large that they become significant components
of long-term carbon budgets (Fig. 6.8; Box 6.1).
Carbon losses during fires in the Canadian
boreal forest, for example, are equivalent to 10
to 30% of average NPP (Harden et al. 2000).
Controls over Net
Ecosystem Production
NEP is determined by factors that cause an
imbalance between carbon gain and loss. An
ecosystem is never at equilibrium at any
moment in time. NEP varies with season, time
since disturbance, interannual variation in
weather, and long-term trends in environment.
High-latitude ecosystems, for example, are a net
carbon source in warm years and a carbon sink
in cool years (Goulden et al. 1998) because heterotrophic
respiration responds to temperature
more strongly than does photosynthesis in cold
climates. We expect NEP to change in response
to long-term changes in any factor that differs in
its effects on GPP and the various avenues of
carbon loss from ecosystems (e.g., plant respiration,
heterotrophic respiration, disturbance,
or leaching loss). Increased concentrations of
atmospheric CO2 or nitrogen inputs to ecosystems,
for example, have greater direct effects on
GPP than on decomposition, whereas a reduction
in the soil moisture of poorly drained
wetlands increases decomposition and fire
probability more strongly than it affects NPP.
Human activities are currently having greatest
impact on precisely those environmental factors
that we expect to affect plants and decomposers
differentially and therefore to affect global terrestrial
carbon storage (see Chapter 15).
Net carbon accumulation by an ecosystem
depends more strongly on time since disturbance
than on climate. The greatest causes of
variation in NEP among ecosystems are cycles